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c#performancelist

Efficiency of List<T>.IndexOf() versus List<T>.FindIndex()

Which one of the methods

  • List<T>.IndexOf() and
  • List<T>.FindIndex()

is more efficient in terms of processing time?

The type of T in this instance is String.

Solution

  • IndexOf performs a for-loop, using the Equals implementation of the objects being searched to look for a match. FindIndex also performs a for-loop but evaluates a Predicate to check for a match instead.

    They each boil down to a for-loop. While they both technically have an O(n) design, the use of a delegate in FindIndex will have some overhead. The difference in performance can be seen in Denis19901's answer. Here are some MSDN excerpts:

    List<T>.IndexOf Method (T):

    This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

    List<T>.FindIndex Method (Predicate<T>):

    This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

    That said, the two functions would be used quite differently. The former assumes you have an object from the list, and you just need to know at what index it exists at (if any) in the list.

    The latter assumes you know some criteria about an object, and you want to find the first index where an object in the list matches that criteria. There could be multiple matches, but the method returns the first match.